Characteristics of the Beaufort Sea High

1JANUARY 2011 S E R R E Z E A N D B A R R E T T 159 Characteristics of the Beaufort Sea High MARK C. SERREZE AND ANDREW P. BARRETT National Snow and Ice Data Center, Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado (Manuscript received 27 January 2010, in final form 12 August 2010) ABSTRACT Characteristics of the Arctic Ocean’s Beaufort Sea high are examined using fields from the NCEP–NCAR reanalysis. At a 2-hPa contour interval, the Beaufort Sea high appears as a closed anticyclone in the long-term annual mean sea level pressure field and in spring. In winter, the Beaufort Sea region is influenced by a pressure ridge at sea level extending from the Siberian high to the Yukon high over northwestern Canada. As assessed from 6-hourly surface winds, the mean frequency of anticyclonic surface winds over the Beaufort Sea region is fairly constant through the year. While for all seasons a strong closed high can be interpreted as the surface expression of an amplified western North American ridge at 500 hPa, there is some suggestion of a split flow, where the ridge linked to the surface high is separated from the ridge to the south that lies within the main belt of westerlies. The Aleutian low in the North Pacific tends to be deeper than normal when there is a strong Beaufort Sea high. In all seasons but autumn, a strong Beaufort Sea high is associated with positive lower-tropospheric temperature anomalies covering much of the Arctic Ocean; positive anomalies are especially pronounced in spring. Seasons with a weak anticyclone show broadly opposing anomalies. A strong high is found to be a feature of the negative phase of the summer northern annular mode, the positive phase of the Pacific–North American wave train, and, to a weaker extent, the positive phase of the summer Arctic dipole anomaly and Pacific decadal oscillation. The unifying theme is that, to varying degrees, the high- latitude 500-hPa ridge associated with the Beaufort Sea high represents a center of action in each teleconnection pattern. 1. Introduction the Siberian coast, across the pole and into the North Atlantic, known as the Transpolar Drift Stream (Fig. 1) A prominent feature of the annual mean sea level (Thorndike and Colony 1982; Colony and Thorndike 1984). pressure (SLP) field for the Arctic Ocean is an anticy- As is widely known, end-of-summer (September) Arc- clone centered north of Alaska, often referred to as the tic sea ice extent has declined over the past few decades. Beaufort Sea high (BSH). In the annual mean, the BSH September 2007 saw the lowest ice extent of the modern appears as a closed surface high embedded within a satellite era (Stroeve et al. 2008). The ice cover is also pressure ridge extending from northeastern Eurasia into thinning (Nghiem et al. 2006; Maslanik et al. 2007b; Kwok northwest Canada. The surface wind field associated with and Rothrock 2009). Simulations from coupled global cli- the BSH, in conjunction with winds associated with the mate models used in the Intergovernmental Panel on Cli- trough of low pressure that extends from the Icelandic mate Change Fourth Assessment Report that include low into the eastern Arctic, largely controls the mean observed increases in atmospheric greenhouse gas con- circulation of the Arctic sea ice cover (Thorndike and centrations consistently show declining September ice Colony 1982). This circulation is characterized by the extent over the period of observations (Stroeve et al. anticyclonic Beaufort gyre and a transport of ice from 2007; Zhang and Walsh 2006). However, viewed as a group, simulated trends are conservative compared to observations (Stroeve et al. 2007). Many factors may be Corresponding author address: Mark C. Serreze, National Snow contributing to rapid ice loss. These include Arctic warm- and Ice Data Center, Cooperative Institute for Research in Envi- ronmental Sciences, Campus Box 449, University of Colorado ing linked to increased concentrations of black carbon Boulder, Boulder, CO 80309-0449. aerosols (Shindell and Faluvegi 2009); increased spring E-mail: [email protected] cloud cover, enhancing the downward longwave radiation DOI: 10.1175/2010JCLI3636.1 Ó 2011 American Meteorological Society Unauthenticated | Downloaded 10/07/21 01:40 AM UTC 160 JOURNAL OF CLIMATE VOLUME 24 and location of the BSH and associated temperature anomalies are expressed with respect to the phase of atmospheric teleconnection patterns highlighted by other authors in the context of declining Arctic sea ice extent. Our analysis focuses on the period 1979–2008. While the NCEP–NCAR fields are available back to 1948, those from 1979 onward correspond to the modern satellite era and are therefore of higher quality. Use is made of surface winds, sea level pressure, 500-hPa height, and 925-hPa temperature. As shown in past studies (e.g., Serreze et al. 2009), basic circulation and tropospheric temperature features from NCEP–NCAR are very simi- lar to those from other reanalyses. The 925-hPa temperatures are preferred over 2-m temperatures, which are strongly influenced by the modeled surface energy budget. 2. Background Interest in links between declining September sea ice extent and atmospheric circulation started in the mid- to late 1990s (e.g., Serreze et al. 1995; Maslanik et al. 1996) and thereafter grew quickly. Rigor et al. (2002) and Rigor FIG. 1. Annual mean SLP over the period 1979–2008 from the and Wallace (2004) provided important insight on in- NCEP–NCAR reanalysis with overlay of mean sea ice velocity vectors for 1979–2006 based on a combination of satellite and buoy fluences of the northern annular mode (NAM) in winter. data (http://nsidc.org/data/nsidc-0116.html). Ice motion is cm s21. The NAM, also known as the Arctic Oscillation, can be viewed as an oscillation of atmospheric mass between the Arctic and middle latitudes. It is in its positive phase when flux at the surface (Francis and Hunter 2006); and altered the zonally averaged surface pressure is high in mid- ocean heat transport (Polyakov et al. 2005; Shimada et al. latitude pressures and low in Arctic latitudes (Thompson 2006). However, as shown in numerous studies (e.g., and Wallace 1998, 2000). The North Atlantic Oscillation Rigor and Wallace 2004; Ogi and Wallace 2007; Wang (NAO), which relates to covariability in the strengths of et al. 2009; see section 2) variability in the atmospheric the Icelandic low and Azores high, is often viewed as the circulation, including the strength and location of the North Atlantic component of the NAM. From about 1970 BSH, has played an especially prominent role. through the mid-1990s, winter indices of the NAM shifted The present paper examines characteristics and vari- from negative to strongly positive. Rigor et al. (2002) show ability of the BSH, using data from the National Centers that as the winter NAM shifted toward the positive state, for Environmental Prediction–National Center for Atmo- there was a retreat of the BSH to the southern Beaufort Sea, spheric Research (NCEP–NCAR) Reanalysis I (Kalnay a more cyclonic motion of ice, and an enhanced transport et al. 1996). It is motivated by recognition that while dif- of ice away from the Siberian and Alaskan coasts, fostering fering points of view have developed regarding the role of openings in the ice cover. While open water in coastal atmospheric circulation anomalies on sea ice conditions areas quickly refroze in response to low-surface air tem- (see section 2), they can find some common ground through peratures, these regions were nevertheless left with an recognition that the BSH projects to varying degrees onto anomalous coverage of young, thin ice in spring, especially several atmospheric modes. We stress that we are not con- prone to melting out in summer. The strongly positive ducting a study of atmosphere–sea ice interactions. Our NAM phase characterizing the period 1989–95 saw strong focus is rather on variability of an atmospheric feature transport of thick, multiyear ice out of the Arctic and into recognized as highly relevant to the sea ice cover. the North Atlantic through the Fram Strait. While the Section 2 provides an overview of known links between NAM subsequently regressed to a more neutral state, the sea ice and the BSH. After evaluating mean seasonal ex- sea ice system may still have memory of these thinning pressions of the BSH in section 3, attention turns to char- processes (Rigor and Wallace 2004). acteristics of the large-scale midtropospheric circulation Other studies have focused on aspects of the summer that favor a strong or weak surface high (section 4). These circulation, and it is in this season that impacts of vari- analyses provide context for assessing how the strength ability in the BSH are especially prominent. Ogi and Unauthenticated | Downloaded 10/07/21 01:40 AM UTC 1JANUARY 2011 S E R R E Z E A N D B A R R E T T 161 FIG. 2. Fields of (a) SLP, (b) SLP anomalies and (c) 925-hPa temperature anomalies averaged for summer (June– August) 2007. Anomalies are with respect to 1979–2008 means. The 925-hPa level gives a more useful assessment of lower-tropospheric warmth over the Arctic Ocean than does the surface temperature, which in summer over the ocean is strongly constrained by the melting sea ice cover. Wallace (2007) find that years with a low September sea (2000), in which the NAM is based on a single EOF anal- ice extent tend to be preceded by anticyclonic summer ysis of geopotential height fields for all calendar months. (July–September) anomalies in the SLP field over the While by their approach, the NAM is most strongly ex- Arctic Ocean, with the core of the anomaly centered at pressed in winter, the NAM as defined by Ogi et al.

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